1. Field of the Invention
The present invention relates in general to a method of electrically connecting an integrated circuit to an electric device.
2. Description of the Prior Art
Liquid crystal display panels, thermal heads, LED arrays and so forth are usually driven by semiconductor integrated circuits which are mounted on separate printed circuit boards. FIG. 1 is a cross sectional view showing a conventional liquid crystal display system. The display comprises a liquid crystal panel 25 and a printed circuit board (called simply PCB for short hereinbelow) 21 connected with the liquid crystal panel 25 by means of flexible plates or rubber connectors 24. The PCB 21 is provided with necessary electrical devices thereon including packaged ICs 22. A problem, however, arises with such displays since the PCB requires a substantial space in the applications of the displays.
An attempt to solve the problem has been to mount IC chips directly on a PCB and to make electrical connection with the printed circuit by means of wire bonding followed by resin moulding protection. The PCB is then connected with a display panel through flexible panels. This configuration is called COB for short. Another approach has been to mount IC chips on a flexible film made of polyimide or the like and electrically connect Au bumps provided on the IC chips with a Cu circuit formed on the film and coated with Au to establish Au-Au contacts. The film provided with the IC chips is directly connected with a display panel in order to supply driving signals. This configuration is called TAB for short.
A further approach has been to mount IC chips directly on a substrate supporting a liquid crystal display device, a thermal head, a LED device or the like. Particularly, in the case of liquid crystal displays, this approach is called COG (chip on glass). A schematic view is illustrated in FIG. 2. A chip 32 is provided with conductive bumps 34 by means of which the chip 32 can make electrical connection with an ITO circuit 31 formed on a glass substrate 30 which is one of a pair of glass substrates between which a liquid crystal material is supported. Alternatively, the chip 32 is mechanically fixed to the glass substrate 30 by means of an organic adhesive 35. The electrical and mechanical connection of a chip 42 with an underlying glass substrate and an ITO circuit 41 can be established also by means of an organic adhesive 45 containing a number of conductive particles 44 as illustrated in FIG. 3. The conductive particles are disposed between and making electrical connection with a contact 43 formed on the chip and a circuit 41 formed on a glass substrate 40.
A serious problem arises with such a technique utilizing IC chips which have not been resin moulded and provided with no lead electrodes extending therefrom. It is often the case that some of the IC chips on the film or the substrate are faulty. In advance of being diced from a wafer, IC chips can be easily tested, e.g. by means of an IC tester. Test of chips can not be carried out after the dicing. Trouble of the chips, however, can occurs also in dicing and subsequent processes. The IC chips must be subjected under test after mounted on the substrate or the film and, if faulty, must be replaced from the substrate or the film by faultless chips. It is, however, very difficult to remove faulty chips from the substrate and correctly mount faultless chip instead. In the case that the chip is fixed by a photo-curable resin which is usually made of a modified acrylate resin, the resin has to be removed by a very strong solvent after completely cured. In the case of that the chip is fixed by an epoxy base photo-curable resin, the cured resin must be mechanically removed by applying thermal stress thereto since there is no solvent to solve the cured resin. In the case of that the chip is fixed by a heat-curable resin, the cured resin must be mechanically removed also by applying thermal stress thereto since there is no solvent to solve the cured resin. In either case, when an IC chip is removed from the substrate or the film, the surface of the substrate or the film tend to be in a such defective condition that fractions of the cured resin yet linger on the surface, that the circuit of the substrate or the film is partially rubbed off and that the surface becomes dirty so that it becomes difficult to make proper electrical connection again between the circuit of the surface and a faultless chip mounted in place of the faulty chip.